The development of air electrodes for metal-air batteries and fuel cells requires new stable catalysts. Recent progress in the development of carbonized conjugated polymers complexes makes it possible to design catalysts with enhanced catalytic properties and stability.
The use of conjugated polymers as the precursors for carbonized materials increases nitrogen content and reduces the costs of the catalysts. Metal complexes of aniline polymerized on the surface of carbons are among the most prospective precursors for carbonized N-doped air electrode catalysts.
Monomers like aniline are high toxic, and are proved carcinogens for animals. It is desirable to replace these precursors with substances that have lower toxicity. The effectiveness of the N-doped carbon catalysts can be improved with the increase of nitrogen content. For this reason the use of polymer complexes with higher concentration of nitrogen is desirable. The structure of the metal complexes influences the catalytic activity too.
Carbonized conjugated polymers are used as a low cost alternative to noble metal catalysts. However the procedure of catalyst preparation is long and requires expensive (inert atmosphere) ovens that decreases rewards of low cost precursors.
Carbonized catalysts have been applied for air electrodes of the fuel cells with acid electrolytes. It was unobvious that these catalysts were suitable for metal-air batteries operated in alkaline media. The application of conducting polymers (such as polyaniline) in basic form can be problematic because of the absence of electronic conductivity of the polymer in alkaline solutions, a more aggressive media can influence the stability too.
A line of technical solutions for using conjugated polymer complexes as precursors for carbonized catalysts is known and provided below.
Finkelshtain et. al. in U.S. Pat. No 2002/0,132,157 suggested polyaniline, polypyrrole polythiophene and polyfuran complexes with transition metals as precursor for air electrode catalysts. This catalysts were used for membrane-electrode assemblies (MEA) of the fuel cells. The complexes have been reduced before inclusion in MEA.
Zelanay and Bashyam in U.S. Pat. No. 7,550,223 proposed polypyrrole, polyaniline, polythiophene metal complexes as the catalysts. These complexes were exploited as the components of the air electrodes after reduction.
Zelanay and Wu in U.S. Pat. No 2011/0,260,119 proposed a Co—Fe composite catalysts, which outer layer was formed by carbonization of the iron-polyaniline complexes at 900° C. in nitrogen. The catalyst was tested in acid solutions and polymer electrolyte fuel cells.
Zelanay and Wu in U.S. Pat. No 2012/0,088,187 demonstrated polyaniline-iron and polyaniline-cobalt catalysts carbonized in 400° C.-1000° C. temperature range. The fuel cell with these air electrode catalysts demonstrated stable performance at about 0.25 A/cm2 for 200 h. The catalysts was tested in proton based electrolytes. Typical procedure of the carbonized catalysts preparation included the following steps:
1) forming a cold aqueous suspension of the carbon and aniline
2) combining the suspension with an oxidant and transition metal containing compound
3) drying
4) heating in the temperature range 400 C.°-1000 C.° in inert atmosphere
5) leaching with acid
6) repeating step 4.
The objective of the current invention is to produce stable catalysts for air electrodes of the metal air batteries, to decrease the toxicity of the precursors for carbonized catalysts, to decrease the cost of equipment for thermal treatment in air, and to simplify the process by eliminating leaching and subsequent heat treatment.